Visual displays that make use of ambient light to illuminate their pixels (most generally reflective) and that produce an image that is indefinitely stable in the absence of electrical input are often referred to as electronic paper, since they mimic some of the most advantageous properties of paper. Just like white paper that reflects and scatters incident light and does not require additional light sources for viewing the images printed upon it, electronic paper displays reflect and scatter ambient light in the white or light colored areas (often image-free areas) and absorb light in the black or dark color areas (often where the image appears). Thus, an electronic paper display can provide images that are viewable in the absence of backlight or pixel emission illumination (e.g., light emitting diode pixels). The absence of backlight makes such displays more pleasing to the eye, since the appearance of an image on such display resembles the appearance of an image on a sheet of paper. Further, since a backlight source is not required for these displays, they can be manufactured in less bulky, thin forms that may also possess some paper-like flexibility.
Electronic paper displays may also be bistable. Bistability refers to the ability of an image to remain stable in the absence of external stimuli (e.g., an applied electric potential). In bistable displays, the states of individual pixels (e.g., whether the pixels are light or dark) remain intact for long periods of time when no external potential is applied to the display. Therefore, images can be stored on bistable displays for a prolonged time without the need for continuous application of power, much like images stored on paper. This makes bistable displays especially appealing for portable-display applications. Further, since power is consumed by bistable displays only when the image is changed, these displays are more economical for some applications than conventional LCD and CRT displays. In CRT displays, for instance, the image needs to be constantly refreshed. While low refresh rates can conserve some power, this often results in flickering of the display and consequent eye strain of the viewer.
The image on the electronic paper display can be changed when desired, allowing a variety of applications for such displays. In one example, such displays serve as “reusable paper” for displaying still images. In other examples, they are used to display real-time moving imagery in video applications.
The first electronic displays with paper-like properties were developed in the 1970s at Xerox's Palo Alto Research Center. These displays, often referred to as “Gyricon” displays, are based on rotation of optically and electrically anisotropic spheres embedded in an elastomer. In one example of a Gyricon display, each sphere is composed of negatively charged black wax or plastic on one side and positively charged white wax or plastic on the other side. Each sphere is suspended in a dielectric fluid contained within a cavity formed in a plasticized elastomer. Each sphere is free to rotate in the fluid so that it could turn with black or white side to the viewer, thus providing a pixel with a black or white appearance. When an appropriate voltage is applied to the electrodes addressing selected spheres, the spheres rotate in accordance with their dipole moment and display an image to the viewer.
Gyricon technology for the most part, however, has only been practically applied to reflective based displays. In general, however, no effective manufacturing process has been developed that produces light valves (balls) with crisp transmissive/opaque regions.
The brightness and contrast of displayed images on these rotating element reflective based displays is primarily determined by the maximum reflectance that a display may attain. The overall reflectance of the display is influenced by the quality of optically and electrically anisotropic spheres as well as by optical properties of the material filling the gaps between individual spheres. Improved reflective versions of gyricon displays, thus, have evolved as described in U.S. Pat. Nos. 5,754,332 issued to Crowley et al., and 5,815,306 issued to Sheridon et al. More recent improvements to the electronic paper displays have further improved the overall reflectance that is comparable to that of paper. One such invention is embodied in our U.S. patent application Ser. No. 11/973,883, to Lipovetskaya and Gobrogge, filed Oct. 9, 2007, and entitled “ELECTRO-OPTIC DISPLAY”, and herein incorporated by reference in its entirety. In general, this new design features an improve matrix having cells arranged in a square or hexagonal close packed patterns which significantly increase the density of the rotating elements, thus, improving reflectance. Another such improvement includes U.S. patent application Ser. No. 11/625,904, to Shieh and Lee, filed Jan. 23, 2007, and entitled “RECONFIGURABLE COLOR SIGNAGE USING BISTABLE LIGHT VALVE”, and herein incorporated by reference in its entirety.
While these more recent improvements to the gyricon displays all offer improved reflectance characteristics, thus improving brightness and contrast, these reflective-based displays are still not comparable to that of transmissive based displays. Accordingly, there is a need for a rotating element type, transmissive based display that can provide bright, high-contrast images. It should be suitable for viewing both still and moving imagery, and should allow fabrication in thin and flexible forms. In addition, such display should preferably be robust and environmentally stable, e.g., it should be capable to withstand high-temperature and high-humidity conditions.